1. Field of the Invention
The present invention relates to an apparatus and method for expanding a service area of a code division multiple access (CDMA) system, and more particularly to an apparatus and method for expanding a service area of a CDMA system restricted in terms of timing.
2. Description of the Prior Art
In a CDMA mobile communication system, all mobile and base stations are temporally synchronized to a reference clock of the CDMA system. A mobile station acquires temporal information from a message transmitted from a base station. Based on the temporal information, the mobile station sets a reference time, thereby being synchronized to the base station. The reference time set by the mobile station is delayed, as compared to that set by the base station, because of a propagation delay of signals transmitted from the base station to the mobile station and a processing delay of signals in the mobile station. The mobile station transmits signals in sync with the reference time set by itself. Such a reverse link signal is received to the base station after being delayed by the propagation delay. Thus, the base station, which transmits signals in sync with a system reference clock, receives reverse link signals delayed for a certain period of time from the system reference clock. Hereinafter, such a time delay is referred to as a xe2x80x9cbidirectional propagation delayxe2x80x9d.
In CDMA systems, the maximum allowable value of such a bidirectional propagation delay is hardwarily restricted by a modem application-specific integrated circuit (ASIC) equipped in a base station. The maximum allowable value of the bidirectional propagation delay is about 416 xcexcs. This value corresponds to a cell radius of about 62 km (416 xcexcs * the velocity of light/2) in an open area.
In the above mentioned conventional CDMA systems, however, the maximum allowable cell radius is restricted in terms of timing by a modem ASIC equipped in a base station. Due to such a restriction in cell radius, a large number of base stations should be installed on a wide area, including thinly populated areas, which is uneconomical.
U.S. Pat. No. 5,511,067 discloses a layered channel element in a base station modem for a CDMA cellular communication system associated with the above mentioned CDMA systems. A channel element is associated with each of five code channels including a pilot channel, a synchronization channel, a paging channel, a traffic channel, and an access channel. The channel element includes a control element and a modem. The pilot and synchronization channels are synchronized to system time by a CDMA system synchronization element, which can comprise, for example, a GPS receiver that provides a point from which the pilot and synchronization channel generation can be offset.
However, this technique has a problem in that the service area may be restricted in terms of timing because pilot and synchronization channels are generated in sync with a system time provided by a single synchronization element.
Therefore, the present invention has been made in view of the above mentioned problems, and an object of the invention is to provide an apparatus and method for expanding a service area of a CDMA system which are capable of expanding the service area restricted in terms of timing due to the hardware of a base station modem ASIC equipped in the system.
In accordance with one aspect, the present invention provides an apparatus for expanding a service area of a CDMA mobile communication system from a first zone resulting from a restriction of the service area occurring in terms of timing to second, third and fourth zones gradually expanding wider than the first zone by a distance not longer than a cell radius allowed by a base station modem application-specific integrated circuit (ASIC) equipped in the system, comprising: a clock generating unit for receiving 10 MHZ, time of day (TOD), and 1-pulse per second (PPS) signals from a global positioning system (GPS) receiver and generating a first even-second clock synchronized to the 1-PPS signal, the clock generating unit also generating second, third, and fourth even-second clocks delayed by respective maximum bidirectional propagation delay values associated with the second, third and fourth zones; a first signal processing unit for receiving the first even-second clock from the clock generating unit, the first signal processing unit serving to, in regard to forward links from an associated base station to mobile stations in the service area, modulate and transmit forward link channels covering all the first, second, third and fourth zones in sync with the first even-second clock while, in regard to reverse links from the mobile stations to the base station, detecting and demodulating only reverse link channels transmitted from mobile stations located in the first zone in sync with the first even-second clock; a second signal processing unit for receiving the second even-second clock from the clock generating unit, the second signal processing unit serving to detect and demodulate only reverse link channels transmitted to the base station from mobile stations located in the second zone in sync with the second even-second clock; a third signal processing unit for receiving the third even-second clock from the clock generating unit, the third signal processing unit serving to detect and demodulate only reverse link channels transmitted to the base station from mobile stations located in the third zone in sync with the third even-second clock; and a fourth signal processing unit for receiving the fourth even-second clock from the clock generating unit, the fourth signal processing unit serving to detect and demodulate only reverse link channels transmitted to the base station from mobile stations located in the fourth zone in sync with the fourth even-second clock.
In accordance with another aspect, the present invention provides a method for expanding a service area of a CDMA mobile communication system from a first zone resulting from a restriction of the service area occurring in terms of timing to second, third and fourth zones gradually expanding wider than the first zone by a distance not longer than a cell radius allowed by a base station modem application-specific integrated circuit (ASIC) equipped in the system, comprising the steps of: receiving, at a clock generating unit, 10 MHZ, TOD, and 1-PPS signals from the GPS receiver and generating, from the clock generating unit, a first even-second clock synchronized to the 1-PPS signal and second, third, and fourth even-second clocks delayed by respective maximum bidirectional propagation delay values associated with the second, third and fourth zones with respect to the first even-second clock; receiving, at first, second, third, and fourth signal processing units, the first, second, third, and fourth even-second clocks while modulating and transmitting pilot, synchronization, paging, and traffic channels covering all the first, second, third and fourth zones in sync with the first even-second clock by the first signal processing unit; detecting and demodulating an access channel transmitted from a mobile station located in the service area by one of the first, second, third, and fourth signal processing units associated with the zone where the mobile station is located, in sync with an associated one of the first, second, third, and fourth even-second clocks; modulating and transmitting a forward traffic channel associated with the zone of the mobile station in sync with the first even-second clock by the first signal processing unit; and detecting and demodulating a reverse traffic channel transmitted from the mobile station in sync with the associated even-second clock by the associated signal processing unit.
In accordance with another aspect, the present invention provides an apparatus for expanding a service area of a CDMA mobile communication system from a first zone resulting from a restriction of the service area occurring in terms of timing to second, third and fourth zones gradually expanding wider than the first zone by a distance not longer than a cell radius allowed by a base station modem application-specific integrated circuit (ASIC) equipped in the system, comprising: a clock generating unit for receiving 10 MHZ, time of day (TOD), and 1-pulse per second (PPS) signals from a global positioning system (GPS) receiver and generating a first even-second clock synchronized to the 1-PPS signal, the clock generating unit also generating second, third, and fourth even-second clocks delayed by respective maximum bidirectional propagation delay values associated with the second, third and fourth zones; a first signal processing unit for receiving the first even-second clock from the clock generating unit, the first signal processing unit serving to, in regard to forward links from an associated base station to mobile stations in the service area, modulate and transmit pilot, synchronization and paging channels covering all the first, second, third and fourth zones and a traffic channel covering the first zone in sync with the first even-second clock while, in regard to reverse links from the mobile stations to the base station, detecting and demodulating only reverse access and traffic channels transmitted from mobile stations located in the first zone in sync with the first even-second clock so as to cover only the first zone; a second signal processing unit for receiving the second even-second clock from the clock generating unit, the second signal processing unit serving to, in regard to forward links from the base station to the mobile stations in the second zone, modulate and transmit the forward traffic channel in a state advanced by a desired delay time corresponding to a delay time for which the second even-second clock is delayed from the first even-second clock while, in regard to reverse links from the mobile stations to the base station, detecting and demodulating only reverse access and traffic channels transmitted from the mobile stations in the second zone in sync with the second even-second clock so as to cover only the second zone; a third signal processing unit for receiving the third even-second clock from the clock generating unit, the third signal processing unit serving to, in regard to forward links from the base station to the mobile stations in the third zone, modulate and transmit the forward traffic channel in a state advanced by a desired delay time corresponding to a delay time for which the third even-second clock is delayed from the first even-second clock while, in regard to reverse links from the mobile stations to the base station, detecting and demodulating only reverse access and traffic channels transmitted from the mobile stations in the third zone in sync with the third even-second clock so as to cover only the third zone; and a fourth signal processing unit for receiving the fourth even-second clock from the clock generating unit, the fourth signal processing unit serving to, in regard to forward links from the base station to the mobile stations in the fourth zone, modulate and transmit the forward traffic channel in a state advanced by a desired delay time corresponding to a delay time for which the fourth even-second clock is delayed from the first even-second clock while, in regard to reverse links from the mobile stations to the base station, detecting and demodulating only reverse access and traffic channels transmitted from the mobile stations in the fourth zone in sync with the fourth even-second clock so as to cover only the fourth zone.
In accordance with another aspect, the present invention provides a method for expanding a service area of a CDMA mobile communication system from a first zone resulting from a restriction of the service area occurring in terms of timing to second, third and fourth zones gradually expanding wider than the first zone by a distance not longer than a cell radius allowed by a base station modem application-specific integrated circuit (ASIC) equipped in the system, comprising the steps of: receiving, at a clock generating unit, 10 MHZ, TOD, and 1-PPS signals from the GPS receiver and generating, from the clock generating unit, a first even-second clock synchronized to the 1-PPS signal and second, third, and fourth even-second clocks delayed by respective maximum bidirectional propagation delay values associated with the second, third and fourth zones with respect to the first even-second clock; receiving, at first, second, third, and fourth signal processing units, the first, second, third, and fourth even-second clocks while modulating and transmitting pilot, synchronization, paging, and traffic channels covering all the first, second, third and fourth zones in sync with the first even-second clock by the first signal processing unit; detecting and demodulating an access channel transmitted from a mobile station located in the service area by one of the first, second, third, and fourth signal processing units associated with the zone where the mobile station is located, in sync with an associated one of the first, second, third, and fourth even-second clocks; modulating and transmitting a forward traffic channel in sync with the first even-second clock by the first signal processing unit when the zone of the mobile station corresponds to the first zone while modulating and transmitting, by the associated signal processing unit, the forward traffic channel in a state advanced by a desired delay time corresponding to a delay time for which the associated even-second clock is delayed from the first even-second clock when the zone of the mobile station corresponds to one of the second, third, and fourth zones; and detecting and demodulating a reverse traffic channel transmitted from the mobile station in sync with the associated even-second clock by the associated signal processing unit.